Yeah, if his past relationship wasnt a few years ago i would have guessed you are a rebound. Maybe you are .. But if the relationship was from that long ago, it seems a little farfetched..
Usually with rebounds, they jump into something serious right away and too fast. Its because thats all they have known in their previous relationship, they miss it, so they try to get it back. Then when they realise they have moved too fast, they will start to pull away, it isnt what they thought it was and will start usually missing their ex and start comparing.
If you are not a rebound, then maybe it is you who is moving a little too fast and he is pulling away? You dont really sound like that is going on though as you stated you know its not serious right now.. And it is him that kind of started calling you girlfriend and such.
I would back off slightly, give him his space and tell him to come back when 'his heart is willing'. lol All the best!
You have all the weapons you need... Now fight!